1. Field of the Invention
The present invention relates to an information terminal device such as a personal computer, mobile phone, etc., and more particularly to a device equipped with a function for reproducing contents on or before an expiration time.
2. Description of the Related Art
Many recent information terminal devices, such as notebook type personal computers, mobile phones, etc., are equipped with a function for reproducing contents received via a communication network or recorded on a recording medium. These devices are very convenient since they enable users to enjoy contents, such as favorite music or images, anywhere.
In the meantime, contents, in which a reproduction valid term (for example, the contents are reproducible only within three days of their downloaded date) or reproduction expiration time (for example, the contents are reproducible on or before, for example, Nov. 30, 2001) is set for the protection of copyright, are now increasing. When such contents are reproduced, in conventional mobile communication terminals, the reproduction valid term or reproduction expiration time set for the contents is compared with the present time of an internal clock. If the present time is within the reproduction valid term or on or before the reproduction expiration time, the contents are reproduced.
However, in general, internal clocks employed in information terminal devices have errors caused by reference oscillators. Accordingly, when the errors are large, the reproduction term of contents cannot be accurately managed. Further, the time of the internal clocks of such devices can be manually adjusted. Therefore, if a user intentionally changes the time of the internal clock, contents remain reproducible even after their expiration time, which makes it impossible to prevent unauthorized use of the contents.
Further, in general mobile communication systems, if an information terminal device is positioned outside the service area of a base station, or if it is within a building or under the ground even inside the service area, it cannot receive system time data transmitted from the base station. Also, while the information terminal device is being transported at a high speed, it cannot sometimes receive the system time data from the base station due to, for example, the fading phenomenon.